This invention relates to a pattern sewing machine provided with a thread trimmer and a thread delivery unit in which after a sewing operation, the thread trimmer trims thread and the thread delivery unit is prohibited from delivering thread till another sewing operation starts. Thread is thus prevented from being wasted in a non sewing operation, thereby eliminating the necessity of trimming unnecessarily delivered thread.
In a known pattern sewing machine, after the sewing operation, needle thread is trimmed. During the sewing operation stitches are made with needle thread and bobbin thread on fabric. In the pattern sewing machine the tension release mechanism of a tension regulator releases needle thread from the tension regulator. In the thread trimmer, a movable knife trims the released needle thread in cooperation with a fixed knife. When the needle thread is trimmed, the needle thread having the length corresponding to the quantity of motion of the movable knife is delivered against the tension applied by a pre-tension assembly. The pre-tension assembly is positioned closer to a thread supply source than to the tension regulator. Therefore, after the needle thread is trimmed, remainder needle thread is delivered from the eye in the needle. The amount of remainder needle thread depends on the tension applied by the pre-tension assembly. When the size and material of the needle thread vary, the frictional resistance between the needle thread and the pre-tension assembly varies, thereby altering the amount of remainder needle thread. If remainder needle thread is too long, the excess needle thread is entangled with the bobbin thread when the next sewing operation starts. If remainder needle thread is too short, the remainder needle thread goes out of the eye in the needle when the next sewing operation starts.
To solve the problem caused by too long or short remainder needle thread, Japanese Laid-open Patent Application No. 63-277089 proposes a pattern sewing machine comprising a thread delivery unit and a thread trimmer. The thread delivery unit delivers a predetermined amount of needle thread toward a needle for forming stitches on fabric. After forming stitches on the fabric, the thread trimmer trims needle thread, and the thread delivery units delivers the predetermined amount of remainder needle thread. Consequently, remainder needle thread has a constant length.
A known pattern sewing machine forms various patterns on fabric corresponding to various pattern data pre-stored in a memory. By mounting the thread trimmer and a required control unit onto the pattern sewing machine, crossover thread is automatically cut. As shown in FIGS. 7A and 7B, when characters A and B are formed on fabric, the crossover thread connects the end of the character A and the beginning of the character B. If in the pattern sewing machine the crossover thread is automatically cut, an operator need not cut unnecessary crossover thread with scissors after a series of patterns are formed, thereby contributing to sewing efficiency. If the thread delivery unit is added to the pattern sewing machine, thread tension can be regulated automatically. The pattern sewing machine thus provided with the thread trimmer and the thread delivery unit still has a problem to solve. Specifically, in the pattern sewing machine, after the crossover thread is automatically cut, a needle lowers into the fabric to form the next pattern, followed by excess needle thread delivered by the thread delivery unit. The excess needle thread has the length corresponding to the distance between the previously formed pattern and the next pattern to form. If the distance between the patterns is short, the crossover thread between the patterns is also short. After the crossover thread is cut, the thread delivery unit delivers short but excess needle thread between the patterns. On the other hand, when the distance between the patterns is large, after forming the pattern, a needle bar is disconnected until the next pattern formation starts. The excess needle thread delivered by the thread delivery unit between the patterns results in a large amount through the entire sewing operation. After the crossover thread is cut, the excess needle thread is unnecessarily delivered. The operator is involved in the time-consuming and troublesome cutting of the excess needle thread with scissors.